Motor vehicles are oftentimes provided with a friction-disc clutch mechanism in their powertrain to transmit power between rotating shafts. These clutches can be found in, for example, automatic transmissions, manual transmissions, and limited slip differentials.
One example of a friction-disc clutch mechanism can be found in an automatic transmission and is known as a multi-disc clutch. These clutches are generally used to help control planetary gear sets and are often actuated by springs and/or fluid pressure. They also generally comprise a series of friction plates separated by a corresponding series of reaction plates that, when tightly pressed together due to the clutch being engaged, rotate cooperatively to achieve a desired output from its associated planetary gear set. The friction plates utilized in such a clutch typically possess a layer or coating of a high-friction material that is bonded to or otherwise present at the plate's primary contact surface. The reaction plates, on the other hand, typically have a smooth contact surface for engagement with the friction plates' high-friction surfaces. And the manner in which these friction and reaction plates engage and disengage can affect transmission shift performance and clutch durability.
For instance, the constant engagement and disengagement of a multi-disc clutch—and thus the repeated interlocking of the friction and reaction plates under substantial compressive forces—can sometimes result in an undesirable stick-slip phenomenon known as “clutch shudder.” Put differently this term refers to the tendency of clutch plates to momentarily stick and then skid against one another in repeated fashion when the clutch is initially engaged or disengaged. And this stick-slip interaction, along with other powertrain resonances and changing slip speeds, can generate a rather annoying noise or chatter and also accelerate degradation of the clutch plates thus shortening their operational lifespan. These clutch shudder occurrences often occur at high clutch operating temperatures, and under high apply pressures and/or low relative velocities (slip speeds) between opposing plate contact surfaces; mostly because these conditions are likely to facilitate the formation of strong local adhesive bonds between the high-friction surface of the friction plate and the smooth contacting surface of the reaction plate. As a result, a spike in the coefficient of friction occurs and causes the contacting surfaces at the friction interface to operate outside of the desired elasto-hydrodynamic (EHD) regime and instead in the boundary lubrication regime. Moreover, instances of clutch shudder can generate unwanted thermal energy that can affect both transmission operation and clutch plate durability.
As such, a number of efforts have been made to try and minimize the occurrence of clutch shudder. For example, friction modifiers or boundary lubricant additives have been added to the lubrication fluid of wet-clutch mechanisms. But these modifiers and additives tend to be expensive and often deteriorate over time. As another example, surface hardening techniques such as nitriding have been used to manufacture stronger, more wear-resistant clutch components. But these processes tend to be expensive, often require the use of large chambers or machines, and are complicated and tedious procedures to perform in conjunction with clutch components.
Thus, there exists a need to develop a smooth-surfaced clutch plate that can interact with a high-friction surface of adjacent clutch plate in a manner that helps decrease the amount and magnitude of plate-to-plate interactions that lead to clutch shudder, noise, and clutch plate wear.